Motion measurement of acoustically levitated object

Abstract

A system is described for determining motion of an object that is acoustically positioned in a standing wave field in a chamber. Sonic energy in the chamber is sensed, and variation in the amplitude of the sonic energy is detected, which is caused by linear motion, rotational motion, or drop shape oscillation of the object. Apparatus for detecting object motion can include a microphone (24) coupled to the chamber and a low pass filter (40) connected to the output of the microphone, which passes only frequencies below the frequency of sound produced by a transducer (18) that maintains the acoustic standing wave field. Knowledge about object motion can be useful by itself, can be useful to determine surface tension, viscosity, and other information about the object, and can be useful to determine the pressure and other characteristics of the acoustic field.

We claim: 
     
       1. A method for determining a frequency of motion of an object that is acoustically positioned in a chamber by an acoustic standing wave field in the chamber, comprising: sensing amplitude modulation of said acoustic standing wave field in said chamber, determining one frequency component of said amplitude modulation which is of a frequency less than the frequency of acoustic energy of said standing wave field, and indicating a frequency of object motion to be the frequency of said one frequency component of said amplitude modulation.   
     
     
       2. The method described in claim 1 including: coupling a transducer to said chamber;   applying electrical energy from an oscillator to said transducer;   coupling a microphone to said chamber, connecting the output of said microphone to said oscillator, and varying the frequency output of said oscillator to a frequency at which the amplitude of said acoustic standing wave field is close to a maximum;   said step of sensing variations including coupling the output of said microphone to a filter that passes only frequencies below the frequency of energy supplied by said oscillator to said transducer.   
     
     
       3. The method described in claim 1 including: establishing a plurality of acoustic standing wave fields in said chamber, each field being resonant to a different dimension of said chamber, and each field having a different frequency;   said step of sensing variations including sensing variations in amplitude of each of said fields, whereby to indicate object motion in each of a plurality of directions.